Ergon weapons are "mythic equivalent" weapons for Rune Fencer and Geomancer implemented in the May 2014 Version Update[1].There never will be plans to implement Ergon weapons for any other job, as these are stand-ins for Rune Fencer and Geomancer's Mythic Weapons.

§Unlike Mythic Weapons at 99 or 119, completed Ergon Weapons lack the 30% damage boost factor when their Ergon Weapon Skill is used in tandem with the weapon[2].§As of the October 2018 Patch, Ergon weapons now possess an Afterglow aura.[3]

Mythic Acquisition

There are a series of quests one must complete in the pursuit of a Mythic Weapon.
Be forewarned that these are laborious tasks which will take a while to complete.